1. Field of the Invention
The present invention relates to a communication system and a communication method which enable communication with a mobile terminal and, more particularly, to a communication system and a communication method which enable comparatively simple information communication, such as short-message communication, using a mobile communication system to be performed with improved reliability in an environment in which the influence of multipath interference, fading or the like upon communication is considerable. The present invention also relates to a communication system and communication method which enable effective use of frequencies for the above-mentioned kind of communications or the like.
2. Description of the Related Art
Various communication systems have been proposed for information communication with a mobile terminal. For example, communication systems described below may relate to the present invention.
A specialized mobile radio (SMR) communication system is a system enabling communication in a large-zone service area by placing a base station at the center of one area (zone) corresponding to the service area. This communication system, however, requires frequencies in a wide band for communication with many users in the large zone.
A cellular communication system has a service area zone divided into a group of a large number of small zones (cells), and has base stations respectively placed in the small zones to perform transmitting and receiving by using different frequencies respectively assigned to the zones. That is, to enable communication in a large-zone service area, a multiplicity of base stations is provided to cover the entire service area. In the cellular system, different frequency bands are used in cells comparatively close to each other, and frequencies in the same band can be used for cells which are located at such distances from each other that jamming therebetween can be ignored. Therefore, the overall frequency band for the cellular system can be smaller than that for the SMR system. However, since the frequency bands for cells which are located so close to each other that jamming can occur, as well as the frequency bands for adjacent cells, are set different from each other, the overall frequency band necessary for the cellular system is substantially wide, though not so wide as that for the SMR system.
Various mobile information communication systems presently used, including the SMR and cellular communication systems, require a disadvantageously wide frequency band, as described above. Available frequency bands are restricted, and there is a limit to use of frequencies in a wide band.
For example, with respect to communication using a simple message, use of frequencies by the SMR communication system or cellular communication system is uneconomical. In particular, the conventional communication systems are being used mainly for speech communication or telephone communication and subordinately for message data and other kinds of data.
In mobile communication, particularly in communication in an urban area, a problem of interference such as multipath cancellation and fading due to the existence of buildings is encountered. To cope with multipath cancellation or fading, trials have been made with a diversity system or the like. If such a system is used, a problem of a considerable increase in facility price arises. In particular, when communication with a mobile terminal is performed, the condition of interference such as multipath cancellation and fading changes continuously, and it is difficult to cope with such interference.
As determined by the present inventors there is a demand for an apparatus (facilities) of a lower price for communication using a particularly simple message data. However, including ways for coping with multipath cancellation and fading will likely increase the facility price. If the apparatus is made without such features due to price controls, the rate of bit error in received data is increased, resulting in a considerable reduction in communication accuracy or, in some case, occurrence of communication failure.
In a multichannel communication system, such as a cellular communication system, using a small-power output, outputs of a multichannel transmitter may be output by being collectively amplified by one wideband amplifier. In other general cases, however, a combiner for combining outputs is used and a duplexer filter or a similar circuit for separating a transmission frequency and reception frequency from each other is also used for the purpose of eliminating mutual interference between channels. However, if such signal processing is performed, a signal loss is caused and the antenna output is considerably reduced. If the power is increased to avoid such an undesirable effect, a need to increase the withstand voltage of electronic circuit components and to provide other various processing techniques arises, resulting in an increase in circuit complexity and an increase in the price of the apparatus.
On the other hand, trails have been made using Internet communication systems. Internet communication systems have been used by being annexed to systems using mobile terminals because of matching in terms of system configuration or because of the relationship with the telephone number system. Considering the efficiency of connection of communication systems, the present connection to Internet communication systems cannot be regarded as suitable.
An object of the present invention is to address the above-described and other problems and provide a communication system and a communication method which are simple in system configuration, but which enable reliable message communication.
Another object of the present invention is to provide a communication system and a communication method which enable uniform communication with a reduced bit error anywhere in urban zones crowded with buildings, suburban zones where buildings are scattered, and so on.
Still another object of the present invention is to provide a communication system in which ain Internet system is incorporated as a xe2x80x9cwiredxe2x80x9d circuit, which can be formed of comparatively low-priced system components, i.e., existing communication and data processing systems and communication apparatus, and which can be easily operated.
A further object of the present invention is to provide a communication system and a communication method arranged to make effective use of frequency bands and to enable communication in a restricted frequency band.
To achieve these and other objects, according to one aspect of the present invention, there is provided a communication system having a first area where communication with a mobile terminal can be performed and a second area where communication with a mobile terminal on the periphery of the first area can be performed. This communication system has a plurality of base stations provided on the periphery of the first area, each of the base stations being capable of communication in the first area, the base stations being capable of communicating with each other in a wireless communication manner. Downlink (down) message data is transmitted on a frequency in a first narrow band from one of the base stations. Each of the other base stations receiving the message data sends out the message on the same frequency as the frequency on which the message data has been received. A mobile terminal which receives a signal transmitted from the originating and other base stations transmits uplink (up) message data to the base stations by using a frequency in a second narrow band different from the frequency for transmission of the down message data.
Transmission of down message data from the base station is performed in the first narrow band of, for example, 6.25 kHz. Each of the other base stations receives this down message data and sends out the received message data on the same frequency as that on which it has received the message data. In this manner, the same message data is sent out from the plurality of base stations, thereby enabling a mobile terminal to receive a receivable one of the transmitted data groups. Thus, a diversity effect is achieved due to the redundancy in transmission. As a result, the influence of multipath interference or fading can be reduced. The mobile terminal receiving the message data sends up message data to the base stations on a frequency in a band different from the first narrow band. As described above, frequencies in different bands are used for transmission of down message data and transmission of up message data. For this communication system, no complicated communication protocols are used.
Preferably, at least one of the plurality of base stations is wire-connected to a wire communication network, and a receiving and repeating mechanism is provided which is wire-connected to the wire communication network, and which receives, in the communication possible area, a signal transmitted from a mobile terminal, and relays the signal via the wire communication network to the base station connected to the wire communication network. When the receiving and repeating means receives data transmitted from the mobile terminal, it sends the received data to the base station via the wire communication network in a wire communication manner.
The receiving and repeating mechanism receives message data transmitted from a mobile terminal when the output level of the mobile terminal is low, when the communication condition is not good, or when up message data cannot be directly transmitted to any of the base stations. The receiving and repeating means sends the data in a wire communication manner via the wire communication network, e.g., an Internet network to the base station connected to the Internet network. As a result, communication can be performed without being affected by multipath interference or fading regardless of the location of the mobile terminal.
More preferably, the originating base station that received the above-mentioned down message monitors the state of transmission of the message between the base stations and transmits a next message data on a frequency in a third narrow band different from the frequencies in the first and second narrow bands. Each of the other base stations receiving the next message data sends out the received message data on the same frequency as the frequency on which the message data has been resolved. A mobile terminal receiving a signal transmitted from the originating and other base stations transmits up message data to the base stations by using a frequency in the second narrow band different from the frequencies for transmission of the above-mentioned down messages.
The next down message data transmission is performed by using a frequency different from the frequency for the preceding down message transmission. Thus, message data transmission is performed at different times by using different frequencies, so that no jamming occurs. Although two narrow frequency bands are used, the width of each frequency band is narrow and the overall band width is limited.
More preferably, a subscriber""s terminal is wire-connected to the wire communication network, and message data from the subscriber""s terminal is transmitted as a down message via the wire communication network from the base station connected to the wire communication network.
In this manner, message data can be transmitted from a subscriber""s terminal.
Specifically, the above-mentioned narrow frequency bands have a width of 6.25 kHz.
Also, specifically, the above-described wire communication network includes an Internet communication network.
According to another aspect of the present invention, there is provided a communication method using a first area where communication with a mobile terminal can be performed, a second area where communication with a mobile terminal on the periphery of the first area can be performed, and a plurality of base stations provided on the periphery of the first area, each of the base stations being capable of communication in the worst area, the base stations being capable of communicating with each other in a wireless communication manner. This communication method includes the steps of transmitting down message data on a frequency in a first narrow band from one of the base stations, sending out, from each of the other base stations receiving the message data, the received message data on the same frequency as the frequency on which the message data has been received, and transmitting, from a mobile terminal receiving a signal transmitted from the originating and other base stations, up message data to the base stations by using a frequency in a second narrow band different from the frequency for the transmission of the down message.
Preferably, at least one of the plurality of base stations is wire-connected to a wire communication network. When communication is performed by providing a receiving and repeating mechanism wire-connected to the wire communication network and capable of receiving in the communication possible area a signal transmitted from a mobile terminal and relaying the signal via the wire communication network to the base station connected to the wire communication network, the receiving and repeating mechanism receiving data transmitted from the mobile terminal sends the received data to the base station via the wire communication network in a wire communication manner.